2.3 mathematical modeling and control of the anaerobic digestion process

369 Pages·2010·4.8 MB·English

THE DESIGN AND CONTROL OF ANAEROBIC PONDS
WITH SPECIFIC REFERENCE TO WESTERN TREATMENT PLANT
Warren Paul
Dip. App. Sc, Grad. Dip, App, Sc,
A thesis submitted inTulfmnent of the requirements
for the degree of Doctor of Philosophy
School of Communications and Informatics
Faculty of Engineering and Science
Victoria University of Technology
Footscray, Melbourne
August 1998
FTS THESIS
628.35 PAU
30001005477254
Pau1. Warren
The design and control of
an,;?erobir oonds with
soecific reference to
To my wife, Julia, for putting up with the seemingly
never-ending story that was this thesis.
Acknowledgments
My thanks to the following people and organisations
• David Hutchison, Terry Scott and Tony Crapper for their support.
• My colleagues and "sounding boards" at Melbourne Water, Brad McLean
and Trevor Gulovsen.
• George Alifraco for rowing across an anaerobic poo pond with me, at three
in the morning, in the name of science.
• Operations staff at Western Treatment Plant for collecting samples of raw
sewage.
• Melbourne Water Corporation for sponsoring this research.
• Dr Keith McLean of CSIRO for his advice during the development of
methanogenic activity tests and anaerobic culture techniques.
• Water ECOscience for performing all chemical analyses of water samples,
• Lim Ong of Probe Analytical for the electron microscopy and X-ray
analyses.
• Mohammed Mohideen and Sue Pepper, at the Centre for Bioprocessing and
Food Technology (Victoria University, Werribee campus), for performing
the anaerobic cultural counts in 1997 and 1998, after my own laboratory at
WTP was closed down in the name of progress.
• My supervisors, Assoc. Prof. Neil Barnett and Assoc. Prof. John Casey, for
their advice and for allowing me the freedom to pursue many different
avenues in the course of this research.
Abstract
This thesis reports results of a study of the control and improvement of
the covered 115E Anaerobic Reactor at Western Treatment Plant. The 115E
Reactor treats 60 ML of sewage per day and produces 8000 mVd of methane
which is harvested for power production. The objectives of this research were
to identify ways to enhance treatment efficiency and reduce variability in
effluent composition from anaerobic ponds. In particular, the research
examined the potential benefits of improved trade waste management and
process control.
Because WTP receives a high proportion of trade wastes, the effects of
various inhibitory and stimulatory chemicals were studied, using a
methanogenic activity test, with a view to increasing the yield of methane in the
longer term. These studies showed that sulphate ion is potentially causing 30%
inhibition of methanogenesis at concentrations found in raw sewage. Calcium
ion, on the other hand, was found to be deficient.
Treatment efficiency could potentially be improved by identifying the
components of the raw sewage that could be inhibitory or stimulatory to the
treatment process and then altering the composition of the sewage, in the longer
term, through changes in regulations dealing with trade wastes. If successful,
this would have a two-fold effect on the operating cost of the Western
Treatment Plant. Firstly, an improvement in anaerobic treatment efficiency
would reduce the power cost associated with aeration in the next stage of the
process. Secondly, improvements in treatment efficiency are concomitant with
an increase in methane produced and, therefore, power produced in the on-site
power plant that uses methane as fuel. In brief, the operating cost of the Plant
would be lowered in the short term by reducing the cost of imported power. In
the future, when covers and power plants are installed on two other lagoons at
WTP, an increase in treatment efficiency would increase revenue from
electricity exports, and this would further offset the operating cost of the Plant.
With regard to reducing variability in effluent composition, both process
regulation and process monitoring techniques were explored as mechanisms for
controlling the anaerobic digestion process.
A feedforward-feedback control equation was developed which
connected the (only) controllable variable, flowrate, to the effluent COD
concentration, raw sewage COD concentration and pond temperature. Using
the control equation, it was demonstrated that there would be insufficient
capacity to divert flows elsewhere within the Plant given the magnitude of the
flow control actions required to keep the pond effluent COD at a fixed target
value. Process monitoring techniques, accepting the daily, weekly and annual
seasonality in the process as normal variability and attempting to identify and
eliminate special causes of variation in order to constrain the variability in
effluent composition, were therefore considered more appropriate to anaerobic
ponds at WTP. This research examined both multivariate and univariate
charting techniques for process monitoring of anaerobic ponds.
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Although monitoring the anaerobic digestion process typically involves
measurement of several related variables including pH, alkalinity, electrode
potential and the concentrations of volatile fatty acids, volatile solids, hydrogen,
methane and carbon dioxide, the multivariate control charts involving serially
correlated data are complicated, difficult to develop and difficult to use. A
study of the dynamics of some of these variables showed that measuring the
effluent COD or methane flowrate would provide as much information as the
suite of other process indicators recommended for anaerobic digestion control.
Monitoring only one variable greatly simplified the development of control
charts for anaerobic ponds. However, even univariate charts can involve
complex time series techniques when the data exhibit serial correlation, and
further work is needed to develop simple procedures that are both statistically
valid and suitable for routine use by operators of wastewater treatment plants.
While not the original intention of this research, certain findings were
made in the course of these studies that could also be used improve the design
of anaerobic ponds. A study of the distribution of methanogens within the
115E Reactor, in order to locate the most appropriate sampling point, revealed a
distinct peak in anaerobic activity midway along the reactor. This was
postulated to correspond to the settlement of suspended and flocculated organic
matter. Further studies showed that the zone of peak activity moved toward the
outlet of the pond as sludge and scum accumulated and, as demonstrated at
WTP, this has serious implications for treatment capacity and gas production if
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"washout" occurs. These observations led to the development of a simple
model that provides an alternative design approach for anaerobic ponds.
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